The present invention relates generally to a tool, and more particularly to a power tool. Still more specifically, the present invention relates to a power tool in which a torque is transmitted from a drive to a tool holder, and in which the transmission or torque is automatically terminated when the torque level exceeds a predetermined value.
The prsent invention will be described hereafter with reference to a power screwdriver by way of example, but should be understood to be applicable to other power tools also.
Power tools, such as power screwdrivers, are already known in which an arrangement is provided for terminating the transmission of torque from the drive to the tool holder (e.g. the chuck holding the drive bit) when the torque acting between the drive and the tool holder exceeds a predetermined value. The prior art proposes a construction wherein the torque transmission takes place via a coupling having a coupling component which is movable axially in the housing of the tool in response to inward displacement of the tool holder which occurs when the housing with the tool accommodated in the tool holder is pressed against a workpiece, for instance a screw which is to be threaded in place. The movement of the coupling component is against the force of a biasing spring which becomes compressed until the axially movable coupling component engages a transverse pin or bolt in the housing. When a predetermined torque level is exceeded, e.g. when the screw has been fully threaded into a workpiece, the transverse bolt is shifted automatically and causes a disengagement of the axially displaceable coupling component with reference to the second component of the coupling, in that it permits an expansion of the heretofore compressed biasing spring.
This prior-art construction performs the desired interruption of the transmission of torque between the drive and the tool holder, but it is quite complicated in a structural sense and requires a relatively large amount of space. This means that the tool according to the prior art is expensive to produce and is rather bulky, the latter being in contradiction to the trend towards smaller and more readily handled tools.